(i) Field of the Invention
This invention is concerned with a process for the recovery of alkali metal hydroxide and acid from the alkali metal salts of monovalent anions which are mixed with the alkali metal salts of multivalent anions using a water-splitting system incorporating bipolar and ion-selective membranes. A simpler version of the process uses an electrodialysis system incorporating cation and anion-selective membranes, for the separation of salts of monovalent anions from the salts of multivalent anions.
More especially this invention relates to the removal of sodium and/or potassium chloride from the electrostatic precipitator (ESP) catch of coastal and/or closed-cycle kraft pulp mills (typically a mixture composed of mostly sodium/potassium sulphate and chloride); and to the concurrent and/or subsequent splitting of such salts into their component acid and base.
This invention also describes approaches through which a partially or totally effluent free (TEF) kraft pulp mill can be achieved in terms of the elements, sodium, sulphur and chlorine.
(ii) Description of Prior Art
The accumulation of sodium chloride in the kraft recovery system, most notably in the particulate matter deposited on the heat-exchanger banks of the recovery boiler, is a problem for mills that use logs floated in sea water. High levels of chloride and potassium in the black liquor can result in low melting point deposits and consequently fire-side corrosion of the superheater tubes in the kraft recovery boiler. Chloride, potassium and sulphide act as melting point depressants and increase the plugging tendency and propensity for corrosion of superheater tubes, by forming a liquid phase in the deposited salt mixture on the tube surface (Backman, R. et al., Tappi Journal, 70:6, 123-127, 1987). The presence of both chloride and potassium can also significantly increase the sintering tendency of the boiler dust at temperatures as low as 400.degree. C. (Skrifvars, B.-J. et al., Tappi Journal, 74:6, 185-189, 1991). To diminish these problems, a portion of the ESP catch is frequently sewered at coastal mills (that use logs floated in sea water) to control chloride levels in the recovery cycle. This control measure also causes a loss of sodium from the liquor cycle. This sodium must be replaced to maintain a mill's sodium/sulphur balance. The significance of this loss may be greater in the future in the case of kraft mills in which chlorine-containing oxidants continue to be used in the bleaching process and bleach plant filtrates are recycled to the kraft recovery cycle (Blackwell, B. and Hitzroth, A., Proceedings of 1992 International Chemical Recovery Conference, pp. 329-350, Seattle, Wash. Jun. 7-11, 1992). It is also possible that potassium and chloride might accumulate in the recovery system of closed-cycle mills employing. TCF (totally chlorine free) bleaching sequences since small amounts of these elements are found in wood and/or process chemicals (e.g. sodium hydroxide).
The factors influencing chloride build-up in the recovery cycle of coastal kraft pulp mills were previously defined (Karjalainen et al., Pulp Paper Mag- Can., 73 (12), 95-101, 1972). These investigators examined the influence of various factors upon the equilibrium concentration of sodium chloride in alkaline pulping liquor and suggested that a simple bleed be used to control the concentration. Various other approaches that can be used to eliminate chloride salts from recovery systems include: processes depending on phase equilibria, chemical precipitation, ion-exchange, electrolytic action, diffusion, and processes involving the removal of chlorine as a gas (Wright, R. H., Pulp & Paper, 31, 56-57, March 1957). Another process recommended in the literature for the removal of chloride from the recovery cycle is one in which all soluble materials are washed out of both unbleached and bleached pulp by countercurrent washing followed by evaporation of all the wash water and burning of the combustible solids (Rapson, W. H., Pulp Paper Mag. Can., 68(12), T635T640, 1967; Rapson, W. H., Pulp Paper Mag. Can., 69(3), T161-T166, 1968). This approach, however, resulted in a high-capital, high-energy operation that produced only a cheap salt by-product. A number of other schemes have been developed to reduce chloride levels in spent cooking liquors from a pulp process, which involve crystallization. Such schemes are disclosed in U.S. Pat. Nos. 3,945,880 to Lukes et al., 3,909,344 to Lukes, 3,954,552 to Lukes et al., 3,746,612 to Rapson et al. and 3,833,462 to Moy et al. Such crystallization schemes involve some loss of active pulping chemicals, and cannot be economically retrofitted to existing mills. U.S. Patent Nos. 3,996,097 to Willard, and 4,000,034 to de Vere Partridge et al. on the other hand, disclose methods wherein chloride salts are removed from the ESP catch through different combinations of standard precipitation and crystallization techniques. These latter two methods require high residual chloride levels in the plant liquors to be effective. In addition, chloride-induced corrosion of the chloride removal equipment utilized in the latter two methods, has been identified as a problem of these approaches. An electrolytic system employing an anion-exchange membrane to remove chloride ion from black liquor was also proposed (Shaw, J. M. and Oloman, C. W., U.S. Pat. No. 4,717,450, Jan. 5, 1988). The problem with this system, however, is that the capital and operating costs are quite high and, furthermore, the anion-selective membrane is subject to fouling by the phenolic-type lignin fragments that are found in black liquor.
Potassium has not been considered to the same extent as chloride in the context of removal from the recovery cycle. However, methods of separation have been examined- The use of ion-exchange resins for potassium removal has been studied in the laboratory. A polystyrene resin impregnated with Cyanex 301 and a crown ether was used to separate potassium from sodium, magnesium and calcium (Belfer, S. et al., Reactive Polymers, 14:1, 81-84 (1991).
Many of the options for chloride and potassium removal have been used industrially. and their comparative performance has been reviewed in a number of papers (Pichon, M. and Muratore, E., ATIP Rev. 31(9), 324-332, 1977; Collins, J. W. and Dallons, V. J., AIChE Symp- Set. 75:190, pp. 263-269, 1979; Christie, R.D., Proceedings of International Pulp Bleaching Conference, Toronto, Ontario, pp. 197-200, June 1979; Venho, J. et al., Proceedings of CPPA Envr. Improvement Conf., Montreal, Quebec, pp. 41-47, October 1976; Uloth, V.C. et al., Proceedings of Pacific Paper Expo, Vancouver, B.C., pp. 42-48, December 1992). 0f all options, leaching of sodium/potassium chloride from ESP catch appears to be the simplest and most economically competitive with the reference option of sewering (Blackwell, B. and Hitzroth, A., Proceedings of 1992 International Chemical Recovery Conference, pp. 329-350, Seattle, Wash. Jun. 7-11, 1992). A potential problem with this option is that appreciable quantities of sodium chloride may have to be present in the ESP catch for efficient leaching to take place.
It is desirable then, to have a cost-effective method of removing chloride and/or potassium from a process solution containing sulphate and other divalent ions., in particular from a solution of ESP catch from a kraft pulping process. Such a method should remove chloride in preference to sulphate and other di- or multivalent anions, so that the latter are not substantially depleted from the ESP catch solution, and are available for recycling to the pulping process. The process should also preferentially remove potassium from the ESP catch solution since high levels of potassium chloride in the black liquor can result in fire-side corrosion of the superheater tubes in the kraft recovery boiler. The process should also preferably be readily retrofitted to existing mills, and not result in severe corrosion problems in the equipment required, and should be capable of operating while maintaining low chloride and potassium levels in the pulp mill process solutions.
Membrane systems involving water splitters in the three-compartment configuration have been recommended for various applications. These include the recovery of fluorine values from fluorosilic acid aqueous streams by electrodialytic water splitting of fluoride salt to hydrofluoric acid and hydroxide base (U.S. Pat. No. 3,787,304 by Chlanda et al.), the recovery of TiO.sub.2 from ilmenite-type ores by digestion, with hydrofluoric acid, in which hydrofluoric acid and ammonium hydroxide are recovered by an electrodialytic water-splitting process from by-product aqueous ammonium fluoride (U.S. Pat. No. 4,107,264 by Nagasubramanian and Liu), the conversion of alkali metal sulfate values, such as sodium or potassium values in spent rayon spin bath liquors, into alkali metal hydroxide and alkali metal sulfate/sulfuric acid (U.S. Pat. No. 4,504,373 by Mani and. Chlanda) and the recovery of metal or ammonium values from materials comprising a salt of a first acid while avoiding formation of gas bubbles in the water-splitting cells. Membrane systems involving water-splitters in the two-compartment configuration have been recommended in a number of applications. These include desalination (U.S. Pat. No. 3,654,125 to Leitz), springing of sulfur dioxide from aqueous sulphite and bisulfite solutions (U.S. Pat. No. 4,082,835 to Chlanda et al.), the removal of alkali metal cations from aqueous alkali metal chloride solutions so as to produce an acidified salt solution and sodium hydroxide (U.S. Pat. No. 4,391,680 to Mani and Chlanda) and the recovery of valuable metal or ammonium values from materials comprising a salt of a first acid while avoiding the formation of gas bubbles (U.S. Pat. No. 4,592,817 to Chlanda and Mani). Moreover, the removal of anions and cations from solutions by electrodialysis is well-documented in the literature (U.S. Patent Nos. 3,673,067, 3,926,759, 4,207,157 and 4,325,792 and 4,715,939).
In none of the aforementioned systems, however, is suggestion made for the application of such membrane systems to the partial or complete recovery of alkali metal hydroxide and acid from the alkali metal salts of monovalent anions which are mixed with the alkali metal salts of multivalent anions as is the case, for example, in kraft pulp mill ESP catch. Moreover, in none of the aforementioned systems is reference made to the electrodialytic separation of ESP catch into its sodium chloride and sodium sulphate components. Even though, when a water-splitting system is used with conventional ion-selective membranes, all the sodium/potassium in the ESP catch can be recovered as sodium/potassium hydroxide, the product acid would be a mixture of hydrochloric and sulphuric acids, and would be difficult to re-use in many mills. In addition, the lost sulphur may be needed to maintain the sodium/sulphur balance in the recovery cycle. By using a monovalent anion-selective membrane, the chloride portion of the ESP catch can be removed with its sodium/potassium value recovered as base. The sodium sulphate which is thus depleted in chloride can also be recovered and used as make-up to the recovery cycle.
Moreover, in none of the afore-mentioned systems is reference made to the integration of these systems into kraft pulp mill operations in terms of achieving a partially or totally effluent free (TEF) mill with respect to the elements sodium, sulphur and chlorine.